Question

I want to use the forward declaration for the ptree class of boost::property_tree.

I use Visual Studio 2010 and boost version 1.48.0.

I do the forward declaration in the following way, in my .h

#ifndef OPTIONS_H_
#define OPTIONS_H_

namespace boost
{
    namespace property_tree
    {
        class ptree;
    }
}

class Options
{
   // something

private:
    boost::property_tree::ptree *m_pxPropertyTree;
};

#endif // OPTIONS_H_

Then, I use the class inside my .cpp

#include <boost/property_tree/ptree.hpp>

using boost::property_tree::ptree;

Options::Options()
{
    m_pxPropertyTree = new ptree();

    // other stuff
}

when I try to compile it, I obtain the following error

error C2371: ‘boost::property_tree::ptree’: redefinition. Different base type. c:\lib\boost\1.48.0\32\boost\property_tree\ptree_fwd.hpp 95

(The error description can be different, I’ve translated it because I’ve the Italian version of Visual Studio).

The line that gives me the error, in ptree_fwd.hpp, is the following

typedef basic_ptree<std::string, std::string> ptree;

Instead, if I don’t use the forward declaration, everything goes well and I compile it successfully.

What I’m doing wrong and how I can use correctly the forward declaration in this case?

Answer 1

Why don’t you just include boost/property_tree/ptree_fwd.hpp? This header contains all forward declaration for the package.

Edit: The solution without the included (which you want to avoid for
good reasons) is to exactly match, what is actually declared.

So:

#include <string>
#include <functional>
namespace boost
{
  namespace property_tree
  {
    template < class Key, class Data, class KeyCompare >
    class basic_ptree;

    typedef basic_ptree< std::string, std::string, std::less<std::string> > ptree;
  }
}